<!DOCTYPE HTML>
<html>
<head>
    <meta charset="utf-8">
    <title>margin</title>
    
</head>

<body>
    <div id="box">
        <h4>css Box modal，h4的margin跟div的margin合并了</h4>
        <div style="background-color:aquamarine;margin:15px; 
        padding:15px;border: 2px solid crimson">
            margin外边距不在background-color覆盖范围。也就是说background对margin不起作用。
        </div>
    </div>

</body>

</html>